The present invention relates to a device for generating a grayscale voltage having a value corresponding to a given grayscale level, and more particularly to a device for generating a grayscale voltage using a serial digital analog converter (DAC).
In recent years, while flat panel displays have attained larger screen size and higher definition, they are also being made increasingly thinner, lighter in weight and lower in cost. In this situation, display drivers are requested to present high-definition display by increasing the number of grayscale levels and providing high-precision, high-resolution grayscale voltages.
FIG. 22A shows an entire configuration of a conventional grayscale voltage generation device 2000. The grayscale voltage generation device 2000 generates grayscale voltages Vlcd(a) to Vlcd(d) having values corresponding to 3-bit display data Data(a) to Data(d) output from latches and applies the generated grayscale voltages to liquid crystal elements (not shown) of a liquid crystal panel via downstream circuits (current drive amplifying circuits in many cases) to thereby drive the liquid crystal panel. The grayscale voltage generation device 2000 includes a voltage divider 20001 and selectors 20002a to 20002d. The voltage divider 20001 is connected to each of the selectors 20002a to 20002d via eight voltage supply lines. The voltage divider 20001 receives a reference voltage Vref, divides the received reference voltage Vref into divided voltages. For the 3-bit data, the voltage divider 20001 generates eight divided voltages for eight grayscale levels.
The voltage divider 20001 and the respective selectors 20002a to 20002d constitute “resistance digital analog converters (R-DACs)”, to generate the grayscale voltages Vlcd(a) to Vlcd(d) corresponding to the display data Data(a) to Data(d).
FIG. 23 shows an internal configuration of the voltage divider 20001 and the selector 20002a shown in FIG. 22A. The voltage divider 20001 includes two resistances having a value of R/2 and eight resistances having a value of R connected like a ladder between the two R/2 resistances. Each of the voltage supply lines is connected at a point between every two adjacent resistances. The selector 20002a includes a switch controller SWC200021 and switches SWa to SWf. The switch controller SWC200021 turns ON/OFF the switches SWa to SWf according to the bit values of the display data Data(a) received from the latch. Specifically, in the selector 20002a, the switch controller SWC200021 selects or does not select the switches SWa to SWf according to the display data Data(a) for one pixel in a tournament manner, to thereby generate an output voltage Vout(a). Such output voltages Vout(a) to Vout(d) are output to the liquid crystal elements in the liquid crystal panel via respective output terminals as the grayscale voltages Vlcd(a) to Vlcd(d).
FIG. 24 shows the relationship between the bit values of the display data Data(a) input into the selector 20002a and the value of the output voltage Vout(a) output from the selector 20002a. As is shown in FIG. 24, by switching the connections of the switches SWa to SWf, it is possible to generate the output voltage Vout(a) having a value that varies with the bit values of the display data Data(a).
As described above, liquid crystal displays (LCDs) adopting the resistance dividing method have an advantage that the circuit configuration can be implemented comparatively easily. Therefore, such LCDs are currently in widespread use as LCDs for notebook PCs.
FIG. 22B shows a grayscale voltage generation device 2100 suited to 4-bit display data Data(a) to Data(d). A voltage divider 21001 of the grayscale voltage generation device 2100 receives the reference voltage Vref and divides the received reference voltage Vref into 16 divided voltages. The voltage divider 21001 therefore includes two resistances having a value of R/2 and 16 resistances having a value of R connected like a ladder between the two R/2 resistances. A total of 16 voltage supply lines are also provided.
As described above, as the number of levels of grayscale of the display data Data(a) to Data(d) is greater (as the number of bits is greater), the number of resistances included in the voltage divider 20001 and the number of voltage supply lines connecting the voltage divider 20001 to each of the selectors 20002a to 20002d must be greater. For example, in the case of 8-bit display data, voltages for 256 grayscale levels (256 voltage supply lines) are necessary. To implement this, an area four times as large as the area occupied by the voltage divider 20001 and the selectors 20002a to 20002d in the case of 3-bit display data is necessary. In the case of 10-bit display data, an area 16 times as large as the area occupied by the voltage divider 20001 and the selectors 20002a to 20002d in the case of 3-bit display data is necessary. This increases the area occupied by semiconductor chips, and thus increases the cost.